1. Field of the Invention
This invention relates to methods for acquisition of seismic data, and, more particularly, to seismic vibrator techniques and specifically deals with the reference signal used to drive a seismic vibrator.
2. Description of the Related Art
In the acquisition of seismic data, seismic waves are used to determine the nature, orientation and location of subsurface formations. In reflection seismic technology, seismic waves are generated at the earth's surface. These waves travel through the earth's crust and the compressional mode of these waves are reflected back to the surface by the various subsurface formations. These reflected waves are detected by means of seismic receivers, or geophones.
This invention focuses on the method of producing the seismic waves. There are various means for production of seismic waves commonly used in the art, which means include but are not limited to explosives and vibrators. Vibrators are used, as their name implies, to vibrate the earth's crust. Their use is attractive as compared to explosives because of their relative safety and the cost. Vibrators, when energized, impart relatively low energy into the earth's crust. Typically, the vibrator operator selects an energization interval, and data are recorded both during the energization interval and a subsequent period during which the vibrator is not energized, but the reflected signals are still being received. This technology, originally developed by Conoco, is referred to in the art as "vibroseis."
With the development of the vibrator in seismic technology came increasing attention to the nature of the signal driving the vibrator. This signal is a controlled wave train, a wave train being a wave which has several cycles. This signal is a sinusoidal vibration of continuously varying frequency. The term for this input wave is "sweep", and a sweep period is commonly several seconds or longer.
Various types of sweeps are possible, each employing some sort of amplitude taper, which is a window function (such as a standard Hanning window) that is applied to the beginning and end of the sweep to insure the amplitude of the sweep smoothly goes to zero at its endpoints. The standard signal is a linear sweep. A linear sweep is a sinusoidal-type signal of essentially constant amplitude envelope wherein the frequency varies linearly with time, either increasing or decreasing monotonically within a given frequency range, yielding a constant sweep rate. A non-linear sweep is a sinusoidal-type signal wherein the frequency varies nonlinearly with time. Typical nonlinear sweeps attempt to compensate for the increased loss or attenuation of higher frequency waves as they travel through the earth by spending more vibration time at the higher frequencies.
In vibration-generated seismics, the field record is correlated with the sweep wave train to produce a correlogram or correlated record. The correlated record resembles a conventional seismic record, as one would receive with an explosive or impulsive seismic source.
It is well known in the seismic art that an undesirable byproduct in vibration-generated seismic signals is side lobe energy. Side lobes are byproducts of the correlation process and lengthen and complicate the desired wavelet. Visually, this appears as small oscillations to either side of the central three lobes of a seismic wavelet. Current methods for acquiring vibrator data, particularly linear sweeps, produce complicated seismic wavelets with excessive amounts of side lobe energy after correlation. This side lobe energy degrades data quality and adversely affects the ability to estimate and control the seismic wavelet in processing. There is therefore a need to generate vibrator data that have a simple wavelet shape and minimal side lobe energy, thus reducing seismic signal distortion and enhancing seismic resolution.
Most vibrator data are acquired using linear sweeps as the reference signal. These data are then correlated with the linear sweep reference signal to produce a record. As stated above, linear sweeps, when correlated or deconvolved, produce complicated wavelets with significant amounts of side lobe energy. Some vibrator data are also acquired using a class of nonlinear sweeps designed to compensate for the increased loss of high frequency waves as they propagate through the earth. These sweeps produce even more complicated wavelets with higher side lobe content than linear sweeps. The large amount of side lobe energy these types of conventional sweeps produce after correlation degrades data quality and adversely affects the ability to estimate and control the seismic wavelet in processing.
Rietsch, E. "Vibroseis Signals with Prescribed Power Spectrum," Geophysical Prospecting, Vol. 25, pp. 613-620 (1977), developed a relationship between a sweep's instantaneous phase function and its power spectral density for sweeps having a constant amplitude envelope, using the fact that a sweep's power spectrum is inversely related to the rate of frequency change of the sweep. Rietsch proposed a method for the determination of an appropriate phase function for a sweep which is to have a certain predetermined power spectrum, noting that the method could be used to design sweeps with autocorrelation functions which had low side lobes. Rietsch at page 617. Therefore, sweeps having predefined power spectra could be designed using this relationship, but vibrator electronic control systems of that time could not reproduce (let alone accurately follow) a user-defined sweep, making this point academic. With the recent advent of a new generation of vibrator control instruments based on advanced microprocessor technology, tight control of the vibrator output force (both amplitude and phase) is now possible. This advancement enables user-defined sweeps to be accurately reproduced and followed by the vibrator. This technological advancement has inspired research into optimal shapes of sweeps, leading to the sweep of this invention.
It is an object of this invention to provide a sweep which, when used in a vibratory seismic system, will produce a signal with minimal side lobe correlation noise.
It is a further object of this invention to provide a sweep which, when used in a vibratory seismic system, will produce a signal which has a simple wavelet shape.
It is a further object of this invention to improve substantially vibratory seismic technology with little or no increase in cost.
These objects, features and advantages of this invention, as well as others, will be more clearly discerned by one of reasonable skill in the relevant art from the specification, figures and claims herein.